Synchronize a PolarDB for MySQL cluster to MaxCompute - Data Transmission Service

You can use Data Transmission Service (DTS) to synchronize data from a PolarDB for MySQL cluster to MaxCompute.

Prerequisites

Activate MaxCompute and create a MaxCompute project. For more information, see Activate MaxCompute and Create a MaxCompute project.
Configure a MaxCompute whitelist to allow DTS to access MaxCompute. For more information, see Configure the whitelist for Alibaba Cloud products to access MaxCompute.
An AccessKey pair for the Alibaba Cloud account that owns the MaxCompute project. See Create an AccessKey pair.
Instead of using the root account AccessKey, you can Create an accountand Set a RAM user as the super administrator for a MaxCompute project.

Limitations

Note

DTS does not synchronize foreign keys from the source database to the destination database. Therefore, cascade and delete operations on the source database are not synchronized to the destination database.

Type	Description
Source database limits	The tables to be synchronized must have a primary key or a UNIQUE constraint, and all fields must be unique. Otherwise, duplicate data may appear in the destination database. If you synchronize data at the table level and need to edit tables (for example, mapping column names), a single synchronization task supports up to 1,000 tables. If you exceed this limit, an error is reported when you submit the task. To resolve this, split the tables into multiple synchronization tasks or synchronize the entire database. Binary logs: You must enable binary logging and set the `loose_polar_log_bin` parameter to `ON`. Otherwise, an error is reported during the precheck and the DTS instance fails to start. For more information about how to enable binary logging and modify parameters, see Enable binary logging and Set cluster and node parameters. Note Enabling binary logging for a PolarDB for MySQL cluster consumes storage space and incurs fees. Binary logs of the PolarDB for MySQL cluster must be retained for at least 3 days. We recommend retaining them for 7 days. Otherwise, the DTS task may fail because DTS cannot obtain the binary logs. In extreme cases, this may cause data inconsistency or data loss. Issues caused by a binary log retention period shorter than the required period are not covered by the DTS Service-Level Agreement (SLA). Note For more information about how to set the retention period for binary logs of a PolarDB for MySQL cluster, see Modify the retention period. Do not run DDL operations that change database or table schemas during schema synchronization or full synchronization. Otherwise, the synchronization task fails. Note During full synchronization, DTS queries the source database. This creates metadata locks that may block DDL operations on the source database.
Other limits	You cannot synchronize data from a read-only node of the source PolarDB for MySQL cluster. You cannot synchronize OSS foreign tables from the source PolarDB for MySQL cluster. Synchronization of INDEX, PARTITION, VIEW, PROCEDURE, FUNCTION, TRIGGER, and FK is not supported. Primary/secondary failover of the database instance is not supported during initial full data synchronization. If a failover occurs, reconfigure the synchronization task. Before you synchronize data, evaluate the performance of the source and destination databases. We recommend synchronizing data during off-peak hours. Otherwise, initial full data synchronization consumes read and write resources on both databases, which may increase the database load. Initial full data synchronization runs concurrent INSERT operations, which causes fragmentation in the destination database tables. As a result, the tablespace of the destination instance is larger than that of the source instance after initial full data synchronization. For table-level data synchronization, do not use tools such as pt-online-schema-change to perform online DDL operations on the synchronization objects in the source database. Otherwise, the synchronization fails. For table-level data synchronization, if no data other than the data from DTS is written to the destination database, you can use Data Management (DMS) to perform online DDL operations. For more information, see Change schemas without locking tables. During DTS synchronization, do not write data other than DTS data to the destination database. Otherwise, data inconsistency between the source and destination databases may occur. For example, if you use DMS to perform online DDL operations while other data is being written to the destination database, data may be lost. Because MaxCompute does not support primary key constraints, duplicate records may appear in MaxCompute if DTS retransmits data due to network issues or other reasons. If a task fails, DTS support staff will attempt to restore it within eight hours. During restoration, they may restart the task or adjust its parameters. Note Only DTS task parameters are modified—not database parameters. Parameters that may be adjusted include those listed in Modify instance parameters.
Other notes	DTS periodically runs the CREATE DATABASE IF NOT EXISTS `test` command on the source database to advance the binary log offset.

Billing

Synchronization type	Pricing
Schema synchronization and full data synchronization	Free of charge.
Incremental data synchronization	Charged. For more information, see Billing overview.

Supported synchronization topologies

One-way one-to-one synchronization
One-way one-to-many synchronization
One-way cascade synchronization
One-way many-to-one synchronization

For more information, see Introduction to data synchronization topologies.

Supported SQL operations

Operation type	SQL statement
DML	INSERT, UPDATE, and DELETE
DDL	ADD COLUMN(ADD COLUMN with attribute columns is not supported)

How it works

Schema synchronization — DTS replicates table schemas from the source database to MaxCompute, appending _base to each table name. For example, the source table customer becomes customer_base in MaxCompute.
Full data synchronization — DTS loads all historical data from the source into the _base tables (called *full baseline tables*). This snapshot is the starting point for incremental sync.
Incremental data synchronization — DTS creates an incremental log table in MaxCompute, named after the destination table with a _log suffix (for example, customer_log), and synchronizes incremental data changes from the source database to this table in real time.
Note
For more information about the structure of the incremental log table, see Incremental log table schema definition.

Procedure

Go to the data synchronization task list page in the destination region. You can do this in one of two ways.
DTS console
1. Log on to the DTS console.
2. In the navigation pane on the left, click Data Synchronization.
3. In the upper-left corner of the page, select the region where the synchronization instance is located.
DMS console
Note
The actual steps may vary depending on the mode and layout of the DMS console. For more information, see Simple mode console and Customize DMS console layout and style.
1. Log on to the DMS console.
2. In the top menu bar, choose Data + AI > DTS (DTS) > Data Synchronization.
3. To the right of Data Synchronization Tasks, select the region of the synchronization instance.
Click Create Task to open the task configuration page.

Configure the source and destination databases.

Category	Setting	Description
N/A	Task Name	DTS automatically generates a task name. We recommend that you specify a descriptive name for easy identification. The name does not need to be unique.
Source Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select PolarDB for MySQL.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the source PolarDB for MySQL cluster is located.
	Cross-account	In this example, the source and destination are in the same Alibaba Cloud account. Select No.
	PolarDB Cluster ID	Select the cluster ID of the source PolarDB for MySQL cluster.
	Database Account	Enter the database account for the source PolarDB for MySQL cluster. The account must have read permissions on the objects being synchronized.
	Database Password	Enter the password for the specified database account.
	Encryption	Select an option based on your requirements. For more information, see Configure SSL encryption.
Destination Database	Select Existing Connection	Select the registered database instance with DTS from the drop-down list. The database information below is automatically configured. Note In the DMS console, this configuration item is Select a DMS database instance. If you have not registered the database instance or do not need to use a registered instance, manually configure the database information below.
	Database Type	Select MaxCompute.
	Connection Type	Select Alibaba Cloud Instance.
	Instance Region	Select the region where the destination MaxCompute project is located.
	Project	Enter the name of the MaxCompute project.
	AccessKey ID of Alibaba Cloud Account	Enter the AccessKey pair that you created in the Prerequisites section.
	AccessKey Secret of Alibaba Cloud Account

After you complete the configuration, click Test Connectivity and Proceed at the bottom of the page.
Note
Ensure that the IP address blocks of the DTS service are added to the security settings of the source and destination databases, either automatically or manually, to allow access from DTS servers. For more information, see Add the IP address whitelist of DTS servers.
Click OK to authorize the ODPS account, and then click Test Connectivity and Proceed again.

Configure the task objects.

On the Configure Objects page, specify the objects to synchronize.

Setting	Description
Synchronization Types	The synchronization types. By default, Incremental Data Synchronization is selected. You must also select Schema Synchronization and Full Data Synchronization. After the precheck is complete, DTS synchronizes the historical data of the selected objects from the source database to the destination cluster. The historical data is the basis for subsequent incremental synchronization.
Naming Rules of Additional Columns	DTS adds extra columns to the destination table in MaxCompute. If an additional column name conflicts with an existing column, the synchronization task fails. Select New Rule or Previous Rule based on your business requirements. Warning Before you select a rule, check whether the names of the additional columns conflict with existing columns in the destination table. Otherwise, the task may fail or data may be lost. For more information about the rules and definitions of additional columns, see Names and definitions of additional columns.
Partition Definition of Incremental Data Table	Select a partition name based on your business requirements. For more information about partitions, see Partitions.
Processing Mode of Conflicting Tables	Precheck and Report Errors: Checks for tables with the same names in the destination database. If any tables with the same names are found, an error is reported during the precheck and the data synchronization task does not start. Otherwise, the precheck is successful. Note If you cannot delete or rename the table with the same name in the destination database, you can map it to a different name in the destination. For more information, see Object name mapping. Ignore Errors and Proceed: Skips the check for tables with the same name in the destination database. Warning Selecting Ignore Errors and Proceed may cause data inconsistency and put your business at risk. For example: If the table schemas are consistent and a record in the destination database has the same primary key or unique key value as a record in the source database: During full data synchronization, DTS retains the destination record and skips the source record. During incremental synchronization, DTS overwrites the destination record with the source record. If the table schemas are inconsistent, data initialization may fail. This can result in only partial data synchronization or a complete synchronization failure. Use with caution.
Capitalization of Object Names in Destination Instance	Configure the case-sensitivity policy for database, table, and column names in the destination instance. By default, the DTS default policy is selected. You can also choose to use the default policy of the source or destination database. For more information, see Case policy for destination object names.
Source Objects	In the Source Objects box, click the objects, and then click to move them to the Selected Objects box. Note You can select objects at the table level.
Selected Objects	To rename a single object in the destination instance, right-click the object in the Selected Objects box. For more information, see Map a single object name. To rename multiple objects in bulk, click Batch Edit in the upper-right corner of the Selected Objects box. For more information, see Map multiple object names in bulk.

Click Next: Advanced Settings.

Setting	Description
Dedicated Cluster for Task Scheduling	By default, DTS uses a shared cluster for tasks, so you do not need to make a selection. For greater task stability, you can purchase a dedicated cluster to run the DTS synchronization task. For more information, see What is a DTS dedicated cluster?.
Retry Time for Failed Connections	If the connection to the source or destination database fails after the synchronization task starts, DTS reports an error and immediately begins to retry the connection. The default retry duration is 720 minutes. You can customize the retry time to a value from 10 to 1,440 minutes. We recommend a duration of 30 minutes or more. If the connection is restored within this period, the task resumes automatically. Otherwise, the task fails. Note If multiple DTS instances (e.g., Instance A and B) share a source or destination, DTS uses the shortest configured retry duration (e.g., 30 minutes for A, 60 for B, so 30 minutes is used) for all instances. DTS charges for task runtime during connection retries. Set a custom duration based on your business needs, or release the DTS instance promptly after you release the source/destination instances.
Retry Time for Other Issues	If a non-connection issue (e.g., a DDL or DML execution error) occurs, DTS reports an error and immediately retries the operation. The default retry duration is 10 minutes. You can also customize the retry time to a value from 1 to 1,440 minutes. We recommend a duration of 10 minutes or more. If the related operations succeed within the set retry time, the synchronization task automatically resumes. Otherwise, the task fails. Important The value of Retry Time for Other Issues must be less than that of Retry Time for Failed Connections.
Enable Throttling for Full Data Synchronization	During full data synchronization, DTS consumes read and write resources from the source and destination databases, which can increase their load. To mitigate pressure on the destination database, you can limit the migration rate by setting Queries per second (QPS) to the source database, RPS of Full Data Migration, and Data migration speed for full migration (MB/s). Note This parameter is available only if Synchronization Types is set to Full Data Synchronization. You can also adjust the rate of full data synchronization when the synchronization instance is running.
Enable Throttling for Incremental Data Synchronization	You can also limit the incremental synchronization rate to reduce pressure on the destination database by setting RPS of Incremental Data Synchronization and Data synchronization speed for incremental synchronization (MB/s).
Whether to delete SQL operations on heartbeat tables of forward and reverse tasks	Choose whether DTS writes heartbeat SQL information to the source database while the instance is running. Yes: Does not write heartbeat SQL information to the source database. The DTS instance may display latency. No: Writes heartbeat SQL information to the source database. This may interfere with source database operations like physical backups and cloning.
Environment Tag	Select an environment tag to identify the instance. Not required for this example.
Configure ETL	Choose whether to enable the extract, transform, and load (ETL) feature. For more information, see What is ETL? Valid values: Yes: Enables the ETL feature. Enter data processing statements in the code editor. For more information, see Configure ETL in a data migration or data synchronization task. No: Disables the ETL feature.
Monitoring and Alerting	Choose whether to set up alerts. If the synchronization fails or the latency exceeds the specified threshold, DTS sends a notification to the alert contacts. No: No alerts are configured. Yes: Configures alerts. You must also set the alert threshold and alert notifications. For more information, see Configure monitoring and alerting during task configuration.

Save the task and perform a precheck.
- To view the parameters for configuring this instance via an API operation, hover over the Next: Save Task Settings and Precheck button and click Preview OpenAPI parameters in the tooltip.
- If you have finished viewing the API parameters, click Next: Save Task Settings and Precheck at the bottom of the page.
Note
- Before a synchronization task starts, DTS performs a precheck. You can start the task only if the precheck passes.
- If the precheck fails, click View Details next to the failed item, fix the issue as prompted, and then rerun the precheck.
- If the precheck generates warnings:
  For non-ignorable warning, click View Details next to the item, fix the issue as prompted, and run the precheck again.
  For ignorable warnings, you can bypass them by clicking Confirm Alert Details, then Ignore, and then OK. Finally, click Precheck Again to skip the warning and run the precheck again. Ignoring precheck warnings may lead to data inconsistencies and other business risks. Proceed with caution.

Purchase an instance.

When the Success Rate reaches 100%, click Next: Purchase Instance.

On the Purchase page, select the billing method and link specifications for the data synchronization instance. For more information, see the following table.

Category	Parameter	Description
New Instance Class	Billing Method	Subscription: You pay upfront for a specific duration. This is cost-effective for long-term, continuous tasks. Pay-as-you-go: You are billed hourly for actual usage. This is ideal for short-term or test tasks, as you can release the instance at any time to save costs.
	Resource Group Settings	The resource group to which the instance belongs. The default is default resource group. For more information, see What is resource management?.
	Instance Class	DTS offers synchronization specifications at different performance levels that affect the synchronization rate. Select a specification based on your business requirements. For more information, see Data synchronization link specifications.
	Subscription Duration	In subscription mode, select the duration and quantity of the instance. Monthly options range from 1 to 9 months. Yearly options include 1, 2, 3, or 5 years. Note This option appears only when the billing method is Subscription.

Read and select the checkbox for Data Transmission Service (Pay-as-you-go) Service Terms.
Click Buy and Start, and then click OK in the OK dialog box.
You can monitor the task progress on the data synchronization page.

Incremental log table schema definition

Before querying the incremental data table, run the following command in MaxCompute to allow full table scans:

set odps.sql.allow.fullscan=true;

Each _log table stores change events from the source. In addition to the original column data, DTS adds the following metadata fields to every row:

Field	Description
`record_id`	Unique identifier for the log entry. Auto-increments for each new entry. For UPDATE operations, DTS generates two entries (before and after the change) with the same `record_id`.
`operation_flag`	The operation type: `I` (INSERT), `D` (DELETE), `U` (UPDATE).
`utc_timestamp`	The operation timestamp in UTC, corresponding to the binary log file timestamp.
`before_flag`	`Y` if the row contains pre-update values; `N` otherwise.
`after_flag`	`Y` if the row contains post-update values; `N` otherwise.

Reading UPDATE events: DTS writes two rows for each UPDATE—one with before_flag=Y (the original values) and one with after_flag=Y (the new values). Both rows share the same record_id.